CNI5e Ch 09

2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*Computer Networks and Internets, 5e

By Douglas E. ComerLecture PowerPoints By Lami Kaya, [email protected]

2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*Chapter 9

Transmission Modes


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*Topics Covered9.1 Introduction 9.2 A Taxonomy of Transmission Modes 9.3 Parallel Transmission 9.4 Serial Transmission 9.5 Transmission Order: Bits and Bytes 9.6 Timing of Serial Transmission 9.7 Asynchronous Transmission 9.8 RS-232 Asynchronous Character Transmission 9.9 Synchronous Transmission 9.10 Bytes, Blocks, and Frames 9.11 Isochronous Transmission 9.12 Simplex, Half-Duplex, and Full-Duplex Transmission 9.13 DCE and DTE Equipment


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.1 Introduction This chapter continues the discussion by focusing on the ways data is transmittedintroduces common terminologyexplains the advantages and disadvantages of parallelismdiscusses the important concepts of synchronous and asynchronous communication


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.2 A Taxonomy of Transmission Modes We use the term transmission mode to refer to the manner in which data is sent over the underlying mediumTransmission modes can be divided into two fundamental categories: Serial one bit is sent at a timeSerial transmission is further categorized according to timing of transmissions Parallel multiple bits are sent at the same timeFigure 9.1 gives an overall taxonomy of the transmission modes discussed in the chapter


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.3 Parallel Transmission Parallel transmission allows transfers of multiple data bits at the same time over separate mediaIn general, parallel transmission is used with a wired medium that uses multiple, independent wiresFurthermore, the signals on all wires are synchronized so that a bit travels across each of the wires at precisely the same timeFigure 9.2 illustrates the concept, and shows why engineers use the term parallel to characterize the wiring


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.3 Parallel Transmission The figure omits two important details:First, in addition to the parallel wires that each carry dataa parallel interface usually contains other wires that allow the sender and receiver to coordinateSecond, to make installation and troubleshooting easythe wires for a parallel transmission system are placed in a single physical cable A parallel mode of transmission has two chief advantages: High speed: it can send N bits at the same timea parallel interface can operate N times faster than an equivalent serial interface Match to underlying hardware: Internally, computer and communication hardware uses parallel circuitry a parallel interface matches the internal hardware well


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.4 Serial Transmission Serial transmissionsends one bit at a timeIt may seem that anyone would choose parallel transmission for high speedsHowever, most communication systems use serial modeThere are two main reasonsFirst, serial networks can be extended over long distances at much less cost Second, using only one physical wire means that there is never a timing problem caused by one wire being slightly longer than anotherSender and receiver must contain a hardware that converts data from the parallel form used in the device to the serial form used on the wireFigure 9.3 illustrates the configuration


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.4 Serial Transmission The hardware needed to convert data between an internal parallel form and a serial form can be straightforward or complexdepending on the type of serial communication mechanismIn the simplest case, a single chip that is known as a Universal Asynchronous Receiver and Transmitter (UART) performs the conversionA related chip, Universal Synchronous-Asynchronous Receiver and Transmitter (USART) handles conversion for synchronous networks


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.5 Transmission Order: Bits and Bytes In serial mode, when sending bits, which bit should be sent across the medium first? Consider an integer: Should a sender transmit the Most Significant Bit (MSB) or the Least Significant Bit (LSB) first?We use the term little-endian to describe a system that sends the LSB firstWe use the term big-endian to describe a system that sends the MSB first Either form can be used, but the sender and receiver must agree


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.5 Transmission Order: Bits and Bytes The order in which bits are transmitted does not settle the entire question of transmission orderData in a computer is divided into bytes, and each byte is further divided into bits (typically 8 bits per byte)Thus, it is possible to choose a byte order and a bit order independentlyFor example, Ethernet technology specifies that data is sent byte big-endian and bit little-endianFigure 9.4 illustrates the order in which Ethernet sends bits from a 32-bit quantity


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.6 Timing of Serial Transmission Serial transmission mechanisms can be divided into three broad categories (depending on how transmissions are spaced in time): Asynchronous transmission can occur at any timewith an arbitrary delay between the transmission of two data items Synchronous transmission occurs continuously with no gap between the transmission of two data items Isochronous transmission occurs at regular intervals with a fixed gap between the transmission of two data items


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.7 Asynchronous Transmission It is asynchronous if the system allows the physical medium to be idle for an arbitrary time between two transmissionsThe asynchronous style of communication is well-suited to applications that generate data at random(e.g., a user typing on a keyboard or a user that clicks on a link)The disadvantage of asynchrony arises from the lack of coordination between sender and receiverWhile the medium is idle, a receiver cannot know how long the medium will remain idle before more data arrivesAsynchronous technologies usually arrange for a sender to transmit a few extra bits before each data item to inform the receiver that a data transfer is startingextra bits allow the receiver to synchronize with the incoming signal the extra bits are known as a preamble or start bits


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.8 RS-232 Asynchronous Character Transmission Consider the transfer of characters across copper wires between a computer and a device such as a keyboardeach data item represents one characterIt is standardized by the Electronic Industries Alliance (EIA)It has become the most widely used for character communicationKnown as RS-232-C, and commonly abbreviated RS-232EIA standard specifies the details, such asphysical connection size (max cable length 50 feet long) electrical details (range between -15v +15v) the line coding being usedIt can be configured to control the exact number of bits per second It can be configured to send 7-bit or 8-bit charactersFigure 9.5 illustrates how voltage varies at different stageswhen a start bit, eight bits of a character, and a stop bit are sent


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.9 Synchronous Transmission A synchronous mechanism transmits bits of data continually with no idle time between bitsafter transmitting the final bit of one data byte, the sender transmits a bit of the next data byteThe sender and receiver constantly remain synchronizedwhich means less synchronization overheadCompare the 8-bit characters on an asynchronous system as illustrated in Figure 9.5 and a synchronous system as illustrated in Figure 9.6 Each character sent using RS-232 requires an extra start bit and stop bitmeaning that each 8-bit character requires a minimum of 10 bit times, even if no idle time is insertedOn a synchronous systemeach character is sent without start or stop bits


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.10 Bytes, Blocks, and Frames If the underlying synchronous mechanism must send bits continuallyWhat happens if a sender does not have data ready to send at all times? The answer lies in a technique known as framing: an interface is added to a synchronous mechanism that accepts and delivers a block of bytes known as a frame To insure that the sender and receiver stay synchronizeda frame starts with a special sequence of bitsMost synchronous systems include an idle sequence (or idle byte) that is transmitted when the sender has no data to sendFigure 9.7 illustrates the concept


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.11 Isochronous Transmission Isochronous transmissionis designed to provide steady bit flow for multimedia applicationsDelivering such data at a steady rate is essential because variations in delay known as jitter can disrupt reception (cause pops or clicks in audio/make video freeze for a short time)Isochronous network is designed to accept and send data at a fixed rate, RNetwork interface is such that data must be handed to the network for transmission at exactly R bits per secondFor example, an isochronous mechanism designed to transfer voice operates at a rate of 64,000 bits per secondA sender must generate digitized audio continuouslyA receiver must be able to accept and play the stream


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.12 Simplex, Half-Duplex, and Full-Duplex Transmission A communications channel is classified as one of three types:(depending on the direction of transfer)SimplexFull-DuplexHalf-Duplex Simplex: a simplex mechanism can only transfer data in a single directionIt is analogous to broadcast radio or televisionFigure 9.8a illustrates simplex communication Full-Duplex: allows transmission in two directions simultaneouslyIt is analogous to a voice telephone conversationin which a participant can speak even if they are able to hear background music at the other endFigure 9.8b illustrates the concept


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.12 Simplex, Half-Duplex, and Full-Duplex Transmission Half-Duplex: A half-duplex mechanism involves a shared transmission mediumThe shared medium can be used for communication in each directionBut the communication cannot proceed simultaneouslyIt is analogous to using walkie-talkies where only one side can transmit at a timeAn additional mechanism is needed at each end of a half-duplex communication that coordinates transmission to insure that only one side transmits at a given timeFigure 9.8c illustrates half-duplex communication


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.13 DCE and DTE EquipmentTerms Data Communications Equipment (DCE) and Data Terminal Equipment (DTE) were originally created by AT&T To distinguish between the communications equipment owned by the phone company and the terminal equipment owned by a subscriberThe terminology persists: if a business leases a data circuit from a phone companythe phone company installs DCE equipment at the businessand the business purchases DTE equipment that attaches to the phone companys equipment


2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.*9.13 DCE and DTE EquipmentFrom an academic point of view, the concept behind the DCE-DTE distinction is not ownership of the equipment Instead, it lies in the ability to define an arbitrary interface for a userIf the underlying network uses synchronous transmissionthe DCE equipment can provide either a synchronous or isochronous interface to the users equipmentFigure 9.9 illustrates the conceptual organizationSeveral standards exist that specify a possible interface between DCE and DTEThe RS-232 standard described in this chapter and the RS-449 standard proposed as a replacement can each be usedIn addition, a standard known as X.21 is also available


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CNI5e Ch 09